Grit and grease are constituents in incoming raw sewage that for best efficiency must be removed before subsequent treatment of the organic materials in sewage. Grit is defined as silica sand and other minerals having a specific gravity greater than one and being non-organic in nature. Grease, while organic in nature, is not amenable to biological treatment as normally employed and early separation of grease and oil facilitates efficient biological processing of the sewage.
Traditionally, a simple detention time gravity sedimentation unit has been used to separate grit as an initial step in treatment of incoming sewage. Grease has been removed downstream in primary clarifiers along with other floatables removed by scum skimmers.
The drawback of removing grease at the clarifier stage downstream of the traditional grit removal tank is that the grease causes severe problems in the service of the clarifiers. In general, grease is hard to remove, and the quicker it is removed from the system, the less trouble can be expected throughout the system.
Therefore, a need presently exists for a combined system which removes the undesirable and readily separable grit and grease as an initial step in sewage treatment.